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                    <h1>闭包测试</h1>
                    
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                         作者 MillZhang
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                          日期 2017-06-11
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                <h3 id="简单一题"><a href="#简单一题" class="headerlink" title="简单一题"></a>简单一题</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">for</span>(<span class="keyword">var</span> i=<span class="number">0</span>;i&lt;<span class="number">5</span>;i++)&#123;<span class="built_in">console</span>.log(i);</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>直接正常循环输出:</p>
<blockquote>
<p>0,1,2,3,4</p>
</blockquote>
<a id="more"></a>
<h3 id="下面这行代码呢"><a href="#下面这行代码呢" class="headerlink" title="下面这行代码呢?"></a>下面这行代码呢?</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">for</span> (<span class="keyword">var</span> i = <span class="number">0</span>; i &lt; <span class="number">5</span>; i++) &#123;</div><div class="line">  setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</div><div class="line">    <span class="built_in">console</span>.log(i);</div><div class="line">  &#125;, <span class="number">1000</span> * i);</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p><code>setTimeout</code>会延迟执行,那么执行到<code>console.log()</code>的时候<code>i</code>已经是5了,所以结果是:</p>
<blockquote>
<p>每隔1秒输出1个5</p>
</blockquote>
<h3 id="同样的类似代码"><a href="#同样的类似代码" class="headerlink" title="同样的类似代码?"></a>同样的类似代码?</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">for</span> (<span class="keyword">let</span> i = <span class="number">0</span>; i &lt; <span class="number">5</span>; i++) &#123;</div><div class="line">  setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</div><div class="line">    <span class="built_in">console</span>.log(i);</div><div class="line">  &#125;, <span class="number">1000</span> * i);</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>只是将上述<code>var</code>换成了<code>es6</code>的<code>let</code>,但输出结果却是:</p>
<blockquote>
<p>每隔1秒分别输出0,1,2,3,4</p>
</blockquote>
<p>此题涉及<code>let</code>与<code>var</code>的<a href="https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Statements/let" target="_blank" rel="external">区别</a></p>
<blockquote>
<p>let声明的变量只在其声明的块或子块中可用，这一点，与var相似。二者之间最主要的区别在于var声明的变量的作用域是整个封闭函数。</p>
</blockquote>
<h3 id="运用闭包获取正常的输出"><a href="#运用闭包获取正常的输出" class="headerlink" title="运用闭包获取正常的输出?"></a>运用闭包获取正常的输出?</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">for</span>(<span class="keyword">var</span> i=<span class="number">0</span>;i&lt;<span class="number">5</span>;i++)&#123;</div><div class="line">  (<span class="function"><span class="keyword">function</span>(<span class="params">i</span>)</span>&#123;</div><div class="line">    setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>&#123;</div><div class="line">      <span class="built_in">console</span>.log(i);</div><div class="line">    &#125;,i*<span class="number">1000</span>);</div><div class="line">  &#125;)(i);</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<blockquote>
<p>每1s分别输出0,1,2,3,4</p>
</blockquote>
<h3 id="上题删掉入参i会如何"><a href="#上题删掉入参i会如何" class="headerlink" title="上题删掉入参i会如何?"></a>上题删掉入参i会如何?</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">for</span> (<span class="keyword">var</span> i = <span class="number">0</span>; i &lt; <span class="number">5</span>; i++) &#123;</div><div class="line">  (<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</div><div class="line">    setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</div><div class="line">      <span class="built_in">console</span>.log(i);</div><div class="line">    &#125;, i * <span class="number">1000</span>);</div><div class="line">  &#125;)(i);</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>内部没有对<code>i</code>保持引用</p>
<blockquote>
<p>每隔1秒输出1个5</p>
</blockquote>
<h3 id="变换一下"><a href="#变换一下" class="headerlink" title="变换一下"></a>变换一下</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div></pre></td><td class="code"><pre><div class="line"><span class="keyword">for</span> (<span class="keyword">var</span> i = <span class="number">0</span>; i &lt; <span class="number">5</span>; i++) &#123;</div><div class="line">  setTimeout((<span class="function"><span class="keyword">function</span>(<span class="params">i</span>) </span>&#123;</div><div class="line">    <span class="built_in">console</span>.log(i);</div><div class="line">  &#125;)(i), i * <span class="number">1000</span>);</div><div class="line">&#125;</div></pre></td></tr></table></figure>
<p>这里给 setTimeout 传递了一个立即执行函数。额，setTimeout 可以接受函数或者字符串作为参数，那么这里立即执行函数是个啥呢，应该是个 undefined ，也就是说等价于：<br><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div></pre></td><td class="code"><pre><div class="line">setTimeout(<span class="literal">undefined</span>, ...);</div></pre></td></tr></table></figure></p>
<p>而立即执行函数会立即执行，那么应该是立马输出的。</p>
<blockquote>
<p>立马输出 0 到 4</p>
</blockquote>
<h3 id="最后一题"><a href="#最后一题" class="headerlink" title="最后一题"></a>最后一题</h3><figure class="highlight js"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div><div class="line">13</div></pre></td><td class="code"><pre><div class="line">setTimeout(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</div><div class="line">  <span class="built_in">console</span>.log(<span class="number">1</span>)</div><div class="line">&#125;, <span class="number">0</span>);</div><div class="line"><span class="keyword">new</span> <span class="built_in">Promise</span>(<span class="function"><span class="keyword">function</span> <span class="title">executor</span>(<span class="params">resolve</span>) </span>&#123;</div><div class="line">  <span class="built_in">console</span>.log(<span class="number">2</span>);</div><div class="line">  <span class="keyword">for</span>( <span class="keyword">var</span> i=<span class="number">0</span> ; i&lt;<span class="number">10000</span> ; i++ ) &#123;</div><div class="line">    i == <span class="number">9999</span> &amp;&amp; resolve();</div><div class="line">  &#125;</div><div class="line">  <span class="built_in">console</span>.log(<span class="number">3</span>);</div><div class="line">&#125;).then(<span class="function"><span class="keyword">function</span>(<span class="params"></span>) </span>&#123;</div><div class="line">  <span class="built_in">console</span>.log(<span class="number">4</span>);</div><div class="line">&#125;);</div><div class="line"><span class="built_in">console</span>.log(<span class="number">5</span>);</div></pre></td></tr></table></figure>
<blockquote>
<p>先看结果:2 3 5 4 1</p>
</blockquote>
<h4 id="why"><a href="#why" class="headerlink" title="why?"></a>why?</h4><p>首先先碰到一个 setTimeout，于是会先设置一个定时，在定时结束后将传递这个函数放到任务队列里面，因此开始肯定不会输出 1 。<br>然后是一个 Promise，里面的函数是直接执行的，因此应该直接输出 2 3 。<br>然后，Promise 的 then 应当会放到当前 tick 的最后，但是还是在当前 tick 中。<br>因此，应当先输出 5，然后再输出 4 。<br>最后在到下一个 tick，就是 1 。</p>
<blockquote>
<p>Promise 的 4 在 1 前面输出是因为 Promise.then()里面的回调属于 microtask, 会在当前 Event Loop 的最后执行, 而 SetTimeout 内的回调属于 macrotask, 会在下一个 Event Loop 中执行</p>
</blockquote>
<p><a href="https://zhuanlan.zhihu.com/p/25407758" target="_blank" rel="external">转自element知乎专栏</a></p>

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                    <ol class="toc"><li class="toc-item toc-level-3"><a class="toc-link" href="#简单一题"><span class="toc-text">简单一题</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#下面这行代码呢"><span class="toc-text">下面这行代码呢?</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#同样的类似代码"><span class="toc-text">同样的类似代码?</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#运用闭包获取正常的输出"><span class="toc-text">运用闭包获取正常的输出?</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#上题删掉入参i会如何"><span class="toc-text">上题删掉入参i会如何?</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#变换一下"><span class="toc-text">变换一下</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#最后一题"><span class="toc-text">最后一题</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#why"><span class="toc-text">why?</span></a></li></ol></li></ol>
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